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Queen's Prize 2019: Refining Ultramarine

Before we get to the doc, there were two sheets full of color samples I prepared from the pigment samples that I had sitting out on the table during the display, but did not include in the written documentation. Scans of both of them are below.

Background

Ultramarine pigment is derived from the
mineral lapis lazuli. The blue color in the stone is produced by lazurite, and
it essentially always has inclusions of calcite, pyrite, and other trace
minerals. The best specimens are a deep blue with minimal amounts of white and
silver inclusions from the calcite and pyrite respectively, while less valuable
samples will have a lighter blue and significant white streaking throughout.
Unless treated, only the best pieces of lapis lazuli can make a passable
pigment. Lesser quality stones will only produce a dull bluish-grey color.

Although lapis lazuli has a vast history of
being used as a decorative stone, there is no evidence of it being ground for
use as a pigment until the sixth century, where it has been found used in
Afghan cave temple wall paintings.[1] The
pigment has also been found to the east in China around the 10th century and
India in the 11th century.[2] It
came into common usage in Europe around the 14th century, where due to the
price of importing the material and the length of the process to prepare it, it
was generally reserved for special uses, such as the robes of Christ and the
Virgin.[3]

What seems to be the most well known method of
preparation for ultramarine pigment is documented by Cennino Cennini in “Il
Libro dell’Arte.” In this, he describes combining ground lapis lazuli with pine
rosin, mastic gum, and wax, and washing the produced substance in lye to render
out the lazurite.[4]
However, this method is not unique to Cennini; several such methods, each using
their own unique yet similar blend of resins and oils, are documented by Jehan
le Begue in a collection of treatises written around the same time period.[5]
While this method was common throughout Europe, the Persians seemed to have
their own method of simply repeating the steps of grinding and washing with a
soap solution until a suitable pigment is produced.[6]

Cennini’s
Method

Converted to step format from D. V. Thompson’s
translation of Il Libro dell’Arte. (See Appendix A for the direct translation.)

Preparation

Obtain some lapis lazuli, a stone with the fewest ash colored inclusions the better.

Over the next three days, knead the plastic a little each day. Linseed oil can be used to prevent sticking to hands.

Form the plastic into two one-foot long sticks for storage.

Extraction

Place the two sticks of plastic in a bowl and fill with warm lye water.

Take one stick in each hand and knead the plastic like you would
bread dough.

When the lye water is saturated with blue, pour it off into a
seperate bowl.

Refill the main bowl with more warm lye and repeat.

When the lye water will no longer turn blue, dispose of the sticks
of plastic.

Order the bowls of blue saturated lye based on when they were
extracted.

Based on how many grades of pigment are desired and keeping in
mind each descending bowl will be less pure than the prior, combine the
bowls. For example, if 18 bowls of lye are obtained and three grades of
pigment are desired, combine the first six bowls, then the next six, and
so on.

Materials

Preparation

Process

While wearing the dust mask and safety glasses, crush the lapis
lazuli in the cast iron mortar, covering the opening of the mortar with a
free hand to prevent too much dust from escaping.

On a glass mulling slab, use a muller to grind the lapis powder
without water.

Sift and repeat until the powder all passes through a 200 mesh
screen.

Take 10.5g pine rosin, 5.25g mastic gum, and 5.25g beeswax,
combine in the pot, and slowly apply heat until the materials melt
together into a liquid, being careful to prevent them from smoking.

Add 28g of powdered lapis and mix thoroughly.

Pour mixture onto a silicone baking mat and allow to cool.

Place the plastic in a bowl of water heated to 110°F.

After a few minutes when the mixture is pliable, remove the
plastic and knead it by pulling the material and folding it back on itself
to work air into the mixture, similar to how taffy is pulled.

Once the plastic has lightened in color (a few minutes of
kneading), form it into a rod and allow to cool for storage. Wait at least
a day before extraction.

Explanation

Steps one through three are essentially
straight out of Cennini’s process, translated to use easily available modern
equivalents of his tools. The only detailing needed in these steps was
specifying how fine of a crush was required. Testing on this happened in
batches four through six, trialing -150+200[7] mesh, -200+325 mesh, and 325
mesh batches of powder. There appeared to be no major changes in quality based
on the mesh size, but the -150+200 sample extracted slower than the other
batches in the first yield. Due to this and the relative pain of pushing the
powder through a 325 mesh screen, I chose 200 mesh to be my cutoff.

Due to the prices of mastic gum I found early
on in this project, I experimented with replacing mastic gum with a pine resin
(made by dissolving pine rosin in turpentine) in batches one and two. These
batches produced passable results, but the material was more difficult to
handle during extraction. The plastic that was created was much stickier and
prone to falling apart. As can be seen by the texture of the pigment in vial
2A, this could be exploited by using a coarser crush of lapis like batches one
and two used before I got my sifters. For ease though, and since I later found
cheaper sources of mastic gum (some Amazon sellers and through Etsy), I would
still recommend using mastic gum and not going this route.

Kneading air into the plastic (steps seven through
nine) is important to the extraction process. I tried a no-knead method in
batch eight and the yield was a third of what a kneaded batch produced. A
theory behind how Cennini’s extraction process works is that water will
preferentially wet the lazurite over the calcite, pyrite, and other impurities,
and pull it into the water.[8] My
theory as to why the kneading is important, and why I call for pulling the
plastic like taffy as opposed to kneading like bread dough in Cennini’s text,
is that the air pockets help the water to flow around the exposed lazurite and
lift it from the plastic. The space the air is occupying means that instead of
the lazurite being held in place by a solid mass of the plastic, the lye
solution only needs to cut through a few strands of the plastic (by converting
the wax to soap) to release the particles.

Extraction

Process

Place the plastic in a bowl of water heated to 110°F.

While the plastic is warming, fill a separate container with ~500
ml of 110°F water.

Add potassium hydroxide to this container until the water is pH
12. (With my local water chemistry, this took .85g per 500ml.)

Set a timer for 15 minutes and put a glove on the hand that will
be doing the kneading.

Transfer the plastic into the lye solution and begin kneading it
like a bread dough, folding it over on itself. The water should take on a
blue tint as time goes on, and a layer of pigment will gather at the
bottom.

Once 15 minutes has elapsed, form the plastic back into a rod,
rinse off any lye solution, and set aside for storage.

Allow the solution to settle over the next several hours to a day,
until the top water is a translucent whitish-yellow color with minimal
amounts of blue.

Pour off the solution, taking care not to disturb the layer of
pigment.

Repeat until almost none of the solution remains.

Fill a separate container with hot water and let it sit so any
bubbles from an aerator on the tap come out.

Roughly pour the hot water onto the pigment bed, so it is all
mixed together. Stir if necessary.

Allow the pigment to settle and pour off the water over the next
few hours to days.

If necessary for accessibility, once there is very little water
left, mix up the pigment and water and transfer it to a shallow vessel for
final drying.

Allow to dry. To ensure a completely dry pigment and speed up the
process, the vessel can be placed in front of a space heater (or other
gently blowing heat source) and the drying pigment can be scraped up and
stirred.

Scrape out the pigment and transfer to a suitable container for
storage.

Explanation

The temperature in steps one and two (and
referenced for kneading during the preparation) came from testing several
different possibilities. 110°F gave a workable plastic that was just barely
sticky. Higher temperatures made the plastic much stickier and using linseed
oil like Cennini called for became a necessity, while lower temperatures made
for a stiffer plastic and shortened the workable time for extractions.

Cennini simply calls for lye in his steps.
This would likely be wood ash lye, which is potassium hydroxide. For batches
one and two, I used a soap making calculator to determine how much lye would be
needed to convert 5.25g of beeswax (the amount contained in the plastic), which
was .5g of potassium hydroxide. However, in these two batches, not all the lye
went towards converting the beeswax because I used linseed oil on my gloves (as
Cennini calls for) to combat the increased stickiness of the plastic from the lack
of mastic gum. For batch three, extraction A, I looked up the pH value of
extracted wood ash lye, which was given as 13. Another calculator I found
called for 4.5g of lye to reach pH 13. (Later testing with a pH meter showed
this produced pH 12.7.) This extraction happened far too quickly and seemed to
carry a lot of debris with the lazurite. For extraction B, I dropped it back to
the known good of .5g lye, and finally for extraction C, I tried adding lye
until it reached pH 12. This batch seemed to be a good balance of speed and
quality.

Since I was using gloves, I did not need to
recoat my hands with linseed oil as often, and eventually dropped it entirely
by the time I hit batch three and started using mastic gum. I figure in
Cennini’s recipe, which did use linseed oil, the pH 13 lye would be quickly
dropped into a lower range by the lye converting the oil, since it is both not
bound up in the plastic and has a higher saponification value than the beeswax.

The extraction steps being 15 minutes was
simply a function of how long it took a container of water to cool in my shop.
During extraction 3B when I first tracked temperature, it took about 15 minutes
for the water to drop from 110°F to 90°F, at which point I noticed the plastic
was significantly harder to knead and stopped. This could be combated by using
a crock pot or other heat source during the extraction, but the breaks do
create convenient batches to produce different grades of pigment as Cennini
calls for.

Washing the pigment in steps ten through
twelve was something I did without considering possible consequences at first,
as I did not think it would be good to have the remnants of a soapy lye
solution mixed in with the pigment when it was used later for painting. I
eventually noticed that some of my later extractions were bluer than my earlier
ones in the same batch, which is the opposite of what should be happening. I
figured that this was due to residual soap keeping the pigment “wet,” enhancing
how blue it was compared to the washed samples. This assumption was tested in
batch 10, which intentionally left out the wash. I found the residual soap did
not wet the pigment, however it did form a skin over the surface of the drying
pigment preventing evaporation. So, an unwashed batch left for several days
would still have residual water while a washed batch might have dried
completely in this time. To ensure consistency between samples, I went back and
re-dried all samples in front of a space heater. In addition to the soap, the
top layer of unwashed batches had a whiter top layer from late settling calcite
particles. So, to both avoid contamination of the pigment and to improve the
quality, I recommend including the washing steps. While Cennini does not call
for it, this recommendation is echoed in other treatises on the making of ultramarine.

In step ten, I note that the hot water should
be left to sit for a minute so any bubbles from an aerator can come out. On
several of my earlier washes, I poured hot water onto the pigment directly from
the tap. The small bubbles combined with the residual soap produced a foam that
trapped a good deal of the lazurite particles. In addition, it seemed that
having the lazurite incorporated into the foam stabilized the bubbles, so the
foam stayed on the surface for several days until I went back and crushed the
bubbles on the side of the container by hand. As this is a rather annoying
process, I highly recommend waiting the extra minute for the bubbles to come
out of solution before adding the water.

Future
Testing

There are a few points that could be useful to
test, but were not covered in this project due to time, materials, or just not
thinking it mattered enough.

Would washing the lapis during the first few crushing steps/discarding
the material that crushes first help reduce calcite/other impurities in the
stone?

In batch seven, I note that the yield was lower in both quality and
quantity than I expected, and guessed that this was due to an incomplete crush
of a rock (i.e. taking the first 28g that passed through the 200 mesh sieve
instead of crushing the whole rock until it passed through the 200 mesh sieve
and then taking 28g from the mix). If the first portion of the rock to be
powdered is mostly calcite, then we might be able to improve the quality of the
rock by discarding that portion. We might also be able to remove calcite by
washing residual powder from the first few steps of crushing, while the
lazurite particles are still large and settle quickly, before the final mulling
and moving on to refining.

Does a lower concentration of lapis powder produce better quality
pigment, or does it just slow down extraction?

I had seen some discussion about wither
Cennini’s process used a 12 oz or a 16 oz pound. My batches used the 16oz pound
and produced passable results. It is possible the 12oz pound might yield better
results due to the lower concentration of lapis particles, meaning the
impurities could be more effectively held while washing out the lazurite.
However, it could also just lead to a slower overall extraction with the same
quality.

Does allowing the plastic to sit for some time after kneading affect
extraction, or can the wait be skipped?

Cennini calls for at least three days before
extracting in his text, although this time is mainly for kneading. Another
treatise detailing a similar method to this calls for one day. I did not test
whether this rest is needed simply because it is so unobtrusive to the process
I did not feel the need to try to cut it out.

Appendix
A: Cennini’s Instructions

On The
Character of Ultramarine Blue, and How to make it.

Chapter
LXII

Ultramarine blue is a color illustrious,
beautiful, and most perfect, beyond all other colors; one could not say
anything about it, or do anything with it, that its quality would not still
surpass. And, because of its excellence, I want to discuss it at length, and to
show you in detail how it is made. And pay close attention to this, for you
will gain great honor and service from it. And let some of that color, combined
with gold, which adorns all the works of our profession, whether on wall or on
panel, shine forth in every object. [p. 36]

To begin with, get some lapis lazuli. And if
you want to recognize the good stone, choose that which you see is richest in
blue color, because it is all mixed like ashes. That which contains least of
this ash color is the best. But see that it is not the azurite stone, which
looks very lovely to the eye, and resembles an enamel. Pound it in a bronze
mortar, covered up, so that it may not go off in dust; then put it on your
porphyry slab, and work it up without water. Then take a covered sieve such as
the druggists use for sifting drugs; and sift it, and pound it over again as
you find necessary. And bear in mind that the more finely you work it up, the
finer the blue will come out, but not so beautifully violet[57] in color. It is
true that the fine kind is more useful to illuminators, and for making
draperies with lights on them.[58] When you have this powder all ready, get six
ounces of pine rosin from the druggists, three ounces of gum mastic, and three
ounces of new wax, for each pound of lapis lazuli; put all these things into a
new pipkin, and melt them up together. Then take a white linen cloth, and
strain these things into a glazed washbasin. Then take a pound of the lapis
lazuli powder, and mix it all up thoroughly, and make a plastic of it, all incorporated
together. And have some linseed oil, and always keep your hands well greased
with this oil, so as to be able to handle the plastic. You must keep this
plastic for at least three days and three nights, working it over a little
every day; and bear in mind that you may keep it in the plastic for two weeks
or a month, or as long as you like. When you want to extract the blue from it,
adopt this method. Make two sticks out of a stout rod, neither too thick nor
too thin; and let them each be a foot long; and have them well [p. 37] rounded
at the top and bottom, and nicely smoothed. And then have your plastic in the
glazed washbasin where you have been keeping it; and put into it about a
porringerful of lye, fairly warm; and with these two sticks, one in each hand,
turn over and squeeze and knead this plastic, this way and that, just as you
work over bread dough with your hand, in just the same way. When you have done
this until you see that the lye is saturated with blue, draw it off into a
glazed porringer. Then take as much lye again, and put it on to the plastic,
and work it over with these sticks as before. When the lye has turned quite
blue, put it into another glazed porringer, and put as much lye again on to the
plastic, and press it out again in the usual way. And when the lye is quite
blue, put it into another glazed porringer. And go on doing this for several
days in the same way, until the plastic will no longer color the lye; and then
throw it away, for it is no longer any good. Then arrange all these porringers
in front of you on a table, in series: that is, the yields, first, second,
third, fourth, arranged in succession; and with your hand stir up in each one
the lye with the blue which, on account of the heaviness of this blue, will
have gone to the bottom; and then you will learn the yields of the blue. Weigh
the question of how many grades of blue you want: whether three or four, or
six, or however many you want; bearing in mind that the first yields are the
best, just as the first porringer is better than the second. And so, if you
have eighteen porringers of the yields, and you wish to make three grades of
blue, you take six of the porringers and mix them together, and reduce it to
one porringer; and that will be one grade. And in the same way with the others.
But bear in mind that if you have good lapis lazuli, the blue from the first
two yields will be worth eight ducats an ounce. The last two yields are worse
than ashes: therefore be prudent in your observation, so as not to spoil the
first blues for the poor ones. And every day drain off the lye from the
porringers, until the blues are dry. When they are perfectly dry, do them up in
leather, or in bladders, or in purses, according to the divisions which you
have. And know that if that lapis lazuli stone was not so very good, or if you
worked the stone up so much that the blue did not come out violet, I will teach
you how to give it a little color.[59] [p. 38] Take a bit of pounded kermes[60]
and a little brazil;[61] cook them together; but either grate the brazil or
scrape it with glass; and then cook them together with lye and a little rock
alum; and when they boil you will see that it is a perfect crimson[62] color.
Before you take the blue out of the porringer, but after it is quite dry of the
lye, put a little of this kermes and brazil on it; and stir it all up well with
your finger; and let it stand until it dries, without sun, fire, or wind. When
you find that it is dry, put it in leather, or in a purse, and leave it alone,
for it is good and perfect. And keep it to yourself, for it is an unusual
ability to know how to make it properly. And know that making it is an
occupation for pretty girls rather than for men; for they are always at home,
and reliable, and they have more dainty hands. Just beware of old women. When
you get around to wanting to use some of this blue, take as much of it as you
need. And if you have draperies with lights on them[63] to execute, it ought to
be worked up a little on the regular stone. And if you want it just for laying
in, it wants to be worked over on the stone very, very lightly, always using
perfectly clear water, and keeping the stone well washed and clean. And if the
blue should get soiled in any way, take a little lye, or clear water; and put
it into the dish, and stir it up well; and you will do this two or three times,
and the blue will be purified entirely. I am not discussing its temperas for
you, because I shall be showing you about all the temperas for all the colors
later on, for panel, wall, iron, parchment, stone, and glass. [p. 39]

Appendix
B: Batch Notes

Labeling
Note

Vials
are labeled with a number and a letter. The number indicates which batch number
it is while the letter indicates the extraction. For example, vial ‘2C’ is from
batch number two and contains the material obtained from the third round of
extraction.

Some
vials have only a number or a number followed by a tick mark. These are control
vials. A number alone is a sample of the ground lapis lazuli used in the batch,
while the tick indicates the sample was washed with water and dried.

All
batches and extractions are assumed to use the standard process I outlined. Any
deviations are noted on each batch.

Batch 1

Question: Can pine resin substitute
for mastic gum?/Proof of concept

Explanation: During my initial searches,
mastic gum was essentially the same price as the higher quality lapis I used at
the end of this project. Due to the cost, I wanted to see if I could substitute
another gum like substance for the mastic gum.

Notes: This batch was initially
kneaded using Cennini’s method, but I switched over to placing the plastic in a
mortar and pestle, adding a small quantity of lye solution, then mashing with
the pestle until the water was blue. The water was then poured off and replaced,
and the process repeated until nothing else was obtained from the plastic.

Yields

1A: 4.15g

Total: 4.15g

Unextracted
pigment/impurities: 23.85g

Batch 2

Question: Can pine resin substitute for
mastic gum?

Explanation: Batch 1 was very sticky and had
to be ground in a mortar and pestle for final extraction. This batch used a
thicker version of pine resin to see if that would produce a more workable
plastic.

Extraction Changes: Extractions A and B were
unknown time, unknown temperature, .5g potassium hydroxide in ~500g water.
Extraction C used the standard process.

Notes: This batch was successful in
that the plastic was more workable and actually produced a very good looking
yield in vial 2A. It was still far sticker than later batches using gum mastic.

Yields

2A: 0.8g

2B: 0.77g

2C: 3.41g

Total: 4.98g

Unextracted pigment/impurities:
23.02g

Batch 3

Question: Control batch/What pH should
the extraction water be?

Explanation: I found a reasonably priced
supplier for mastic gum, so this batch was intended to be as close to Cennini’s
process as possible. To that end, I aimed for a pH of 13 for the extraction
water (actual was 12.7) to replicate wood ash extracted lye.

Control Vial: 325 mesh lapis washed with
water, drained, and dried.

Preparation Changes: 325 mesh lapis was used.

Extraction Changes: Extraction A used 4.5g lye in
its water, extraction B used .5g lye, and both were unknown time and
temperature. Extraction C was used to determine the standard of 110°F water,
15 minutes, and pH 12.

Notes: During extraction A, the
plastic was attacked very quickly by the lye and produced a low quality
pigment.

Yields

3A: 1.9g

3B: 0.31g

3C: 2.03g

Total: 4.24g

Unextracted
pigment/impurities: 23.76g

Batch 4

Question: What is the proper size to
grind the lapis lazuli to?

Explanation: Cennini is not detailed in
how fine the powder should be worked up, only that finer is not as vibrant of a
blue but is more useful for some cases. Ayisha also mentioned that she thought
if you ground ultramarine too fine, it would lose its color.

Plastic Changes: 325 mesh lapis was used.

Extraction Changes: None.

Notes: Lapis powder is a pain to
push through a 325 mesh sieve.

Yields

4A: 1.2g

4B: 1.06g

4C: 2.42g

4D: 1.43g

4E: 1.4g

Total: 7.51g

Unextracted
pigment/impurities: 20.49g

Batch 5

Question: What is the proper size to
grind the lapis lazuli to? (continued)

Notes: The extracted pigment was
extremely heavy and settled immediately after extraction, making for a very
quick draining and washing process. This batch was noticeably stickier than
batch 4, likely due to the less even distribution of lapis powder through the
grind size.

Yields

5A: .95g

5B: 1.77g

5C: 2.77g

Total: 5.49g

Unextracted
pigment/impurities: 22.51g

Batch 6

Question: What is the proper size to
grind the lapis lazuli to? (continued)

Notes: The pigment was heavier, but
still required time to settle. The plastic was about as workable as batch 4’s,
despite the coarser grind.

Yields

6A: 2.83g

6B: 1.57g

Total: 4.4g

Unextracted
pigment/impurities: 23.6g

Batch 7

Question: What effect does washing have
on the pigment?

Explanation: At this point, I realized I
was being inconsistent with my washing of the pigment and Cennini’s process
does not call for washing. I suspected not washing allowed residual soap to
“wet” the pigment after it dried, helping it maintain a more vibrant blue.

Preparation Changes: 325 mesh lapis was used.

Extraction Changes: None

Notes: I changed up my plans for
this batch after the fact and still washed it. This is because my yield was
unexpectedly low and pale compared to batch 4, which followed the same process.
I realized that the lapis powder I used in this batch was not part of a completely
crushed stone. That is, some parts of the whole rock I crushed was still in the
150 mesh sieve, some in the 200 mesh, and so on. This means that more of the
softer material that crushed first would have made it into this batch. Since
lazurite has a mohs hardness of 5-5.5, while calcite is only 3, the sample I
used had a much higher concentration of calcite than the overall batch.

Yields

7A: 0.99g

7B: 1.73g

Total: 2.72g

Unextracted
pigment/impurities: 25.28g

Batch 8

Question: Is kneading the plastic
necessary before extraction?

Explanation: While not a particularly
painful step in the process, it does require some active time to work the
material.

Preparation Changes: 325 mesh lapis was used.

Extraction Changes: None.

Notes: Extraction A produced a third
of the material that batch 4 did, with no real change in quality. To confirm no
other issues were confounding the results like in batch 7, I kneaded the
plastic before extraction B, which caused a jump in the yield.

Yields

8A: 0.39g

8B: 3.03g

Total: 3.42g

Unextracted
pigment/impurities: 24.58g

Batch 9

Question: How much of a difference does
the initial quality of lapis lazuli matter?

Explanation: There are two possibilities
for how the extraction process works. Either the process is essentially perfect
and the lazurite content of the initial stone only determines the final yield
quantity, or the process will reduce the amount of impurities but never render
a fully clean material.

Preparation Changes: None, although a higher
quality of lapis lazuli was used than in prior batches.

Extraction Changes: None.

Notes: This batch produced a much
larger quantity and better quality of pigment than previous batches, indicating
the quality of the final pigment is highly dependant on the initial stone.

Yields

9A: 3.15g

9B: 5.8g

9C: 4.62g

9D: 2.5g

9E: 2.6g

Total: 18.67g

Unextracted
pigment/impurities: 9.33g

Batch 10

Question: What effect does washing have
on the pigment? (Take 2)

Explanation: See batch 7 for explanation.
This was another attempt to answer that question, with uncontaminated lapis.

Preparation Changes: None, although a higher
quality of lapis lazuli was used than in prior batches.

Extraction Changes: Steps 10-12 (washing) were skipped.

Notes: As the pigment dried, it
formed a whitish layer on the surface composed of soap residue and some calcite
that escaped the plastic and was in the water. These chunks can be seen in the
final bottled pigments. What this soap cap did was seal in the moisture from
evaporating, so while a washed batch might be dry in a few days, the unwashed
batches would still contain moisture, helping them maintain their more vibrant
blue. After stirring the pigment and placing it in front of a heater to dry, it
became similar in color to batch 9.

Yields

10A: 3.14g

10B: 4.53g

10C: 4.66g

10D: 3.69g

10E: 2.78g

Total: 18.8g

Unextracted
pigment/impurities: 9.2g

Bibliography

Al-Husaini Ahmad Ibn. Calligraphers and Painters: A
Treatise by Qadi Ahmad, Son of Mir-Munshi (circa A.H.1015/A.D.1606) Translated
from the Persian by V. Minorsky, with an Introduction by B.N. Zakhoder,
Translated from the Russian by T. Minorsky, 1959.

Merrifield, Mary Philadelphia. Original
Treatises: Dating from the XIIth to XVIIIth Centuries on the Arts of Painting,
in Oil, Miniature, Mosaic, and on Glass; of Gilding, Dyeing, and the
Preparation of Colours and Artificial Gems; Preceded by a General Introduction;
with Translations, Prefaces, and Notes, 1849.

[5]Mary Philadelphia
Merrifield, Original
Treatises: Dating from the XIIth to XVIIIth Centuries on the Arts of Painting,
in Oil, Miniature, Mosaic, and on Glass; of Gilding, Dyeing, and the
Preparation of Colours and Artificial Gems; Preceded by a General Introduction;
with Translations, Prefaces, and Notes, 1849, 96–111.

[6]Al-Husaini Ahmad
Ibn, Calligraphers and
Painters: A Treatise by Qadi Ahmad, Son of Mir-Munshi (circa A.H.1015/A.D.1606)
Translated from the Persian by V. Minorsky, with an Introduction by B.N.
Zakhoder, Translated from the Russian by T. Minorsky, 1959, 196–197.

[7] This notation implies that at least 90% of
the material will pass through the negative sieve number and will be retained
by the positive sieve. In this case, -150+200 mesh material means that at least
90% will pass through a 150 mesh sieve, but will not pass through a 200 mesh
sieve.

[8]Plesters, “Ultramarine Blue, Natural
and Artificial,” 63.

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